A number of hydrocarbons, especially lower-boiling light hydrocarbons, in formation fluids or natural gas are known to form hydrates in conjunction with the water present in the system under a variety of conditions—particularly at a combination of lower temperature and higher pressure. The hydrates usually exist in solid forms that are essentially insoluble in the fluid itself. As a result, any solids in a hydrocarbon or natural gas fluid are at least a nuisance for production, handling and transport of these fluids. It is not uncommon for hydrate solids (or crystals) to cause plugging and/or blockage of pipelines or transfer lines or other conduits, valves and/or safety devices and/or other equipment, resulting in shutdown, loss of production and, more seriously, risk of explosion, compromised safety of operating personnel and unintended release of hydrocarbons into the environment either on-land or off-shore. Accordingly, hydrocarbon hydrates have been of substantial interest as well as concern to many industries, particularly the petroleum and natural gas industries.
Hydrocarbon hydrates are clathrates, and are also referred to as inclusion compounds. Clathrates are cage structures formed between a host molecule and a guest molecule. A hydrocarbon hydrate generally is composed of crystals formed by water host molecules surrounding the hydrocarbon guest molecules. The smaller or lower-boiling hydrocarbon molecules, particularly C1 (methane) to C4 hydrocarbons and their mixtures, are more problematic because it is believed that their hydrate or clathrate crystals are easier to form. For instance, it is possible for ethane to form hydrates at as high as 4° C. at a pressure of about 1 MPa. If the pressure is about 3 MPa, ethane hydrates can form at as high a temperature as 14° C. Even certain non-hydrocarbons such as carbon dioxide, nitrogen and hydrogen sulfide are known to form hydrates under the proper conditions. Hydrate-forming guest molecules include, but are not necessarily limited to, methane, ethane, ethylene, acetylene, propane, propylene, methylacetylene, n-butane, isobutane, 1-butene, trans-2-butene, cis-2-butene, isobutene, butene mixtures, isopentane, pentenes, natural gas, carbon dioxide, hydrogen sulfide, nitrogen, oxygen, argon, krypton, and xenon.
There are two broad chemical techniques to overcome or control the hydrocarbon hydrate flow hazards, namely thermodynamic and kinetic. The thermodynamic approach is to prevent hydrate formation by addition of “antifreeze” to the production fluids. Suitable thermodynamic hydrate inhibitors include, but are not necessarily limited to, methanol (MeOH), monoethylene glycol (MEG), diethylene glycol (DEG), triethylene glycol (TEG), propylene glycol (PG), certain salts, and mixtures thereof.
The kinetic approach generally attempts (a) to prevent the smaller hydrocarbon hydrate crystals from agglomerating into larger ones (known in the industry as an anti-agglomerate and abbreviated AA) and/or (b) to inhibit, retard and/or prevent initial hydrocarbon hydrate crystal nucleation; and/or crystal growth (known in the industry as a kinetic hydrate inhibitor and abbreviated KHI). Thermodynamic and kinetic hydrate control methods may be used in conjunction.
Kinetic efforts to control hydrates have included the use of different materials as inhibitors. For instance, onium compounds with at least four carbon substituents are used as AA to inhibit the plugging of conduits by gas hydrates. Additives such as polymers with lactam rings have also been employed as KHI to control clathrate hydrates in fluid systems. All these kinetic inhibitors are commonly labeled as Low Dosage Hydrate Inhibitors (LDHI) in the art. KHIs and even LDHIs are relatively expensive materials, and it is always advantageous to determine ways of lowering the usage levels of these hydrate inhibitors while maintaining effective hydrate inhibition. Other suitable KHIs include, but are not necessarily limited to, dendrimeric compounds, hyperbranched polymers, and linear polymers and copolymers. Dendrimeric compounds have also been referred to as “starburst conjugates”. Such compounds are described as being polymers characterized by regular dendrimeric (tree-like) branching with radial symmetry. Suitable but non-restrictive examples of dendrimeric compounds include HYBRANE® polymers available from DSM. One suitable, but non-limiting linear polymer is polyvinylcaprolactam (PVCap). Suitable onium compounds include, but are not limited to, ammonium compounds, and phosphonium compounds, including, but not necessarily limited to those of U.S. Patent Application Publication 2005/0261529 A1, incorporated herein in its entirety by reference.
In the production of hydrocarbons, particularly natural gas, it is common for a wellhead platform (WHP) to be offshore and transport hydrocarbon through a subsea production pipeline “leg” which may join one or more other legs forming a larger pipeline to take the gas to another offshore facility, such as a slug catcher or the like. When the larger pipeline is submerged in the sea, the combination of lower temperatures with high pressures make the pipeline susceptible to gas hydrate formation which may cause the transport problems discussed above. It will be remembered that the flow in a hydrate-susceptible pipeline is multiphase that is both gas phase and liquid phase water and hydrate-forming guest molecules are present.
It is complicated if two or more hydrocarbon production pipeline legs join together to form the subsea pipeline that is susceptible to hydrate formation. It is not a simple matter to adjust the hydrate inhibitor in each leg that joins the subsea pipeline because the combined effect of the types and amounts of hydrate inhibitors may nor may not continue to inhibit hydrates. Further, when the sea temperature drops seasonally, while more of the present hydrate inhibitor may be used, and/or an additional hydrate inhibitor is injected, when warmer weather resumes and the sea gradually warms up it is not a simple or easy matter to then reduce the amounts and types of hydrate inhibitors used. That is, due to complex interactions of different additive types, as will be further explained below, reducing the THI and KHI to levels previously effective may, in fact, not be sufficiently effective, and may actually increase the tendency of hydrates to form. Because hydrate inhibitors are generally expensive, operators have an economic incentive not to use any more inhibitor than necessary. In a multiple tie-in well system (two or more hydrocarbon production pipeline legs joining a sub-sea hydrate-susceptible pipeline), it is often not apparent how to reduce the amounts and types of hydrate inhibitors used without incurring an increased risk of hydrate formation. Assuring smooth flow in these pipelines is very important since interruptions in flow can incur costs of millions of dollars.
Thus, it is desirable that new methods of transitioning between using multiple types of hydrate inhibitors in pipelines to using just one hydrate inhibitor be developed so that effective levels of hydrate inhibition may be maintained while using a minimum amount of hydrate inhibitor to reduce additive costs.